The present invention relates to a process and system for drying organic-contaminated sludge prior to vitrification thereof. The process provides effective heat transfer and destroys organic constituents which can inhibit encapsulation through vitrification.
The effective disposal of hazardous/radioactive waste materials is a continuing problem for industry. These waste materials can take a variety of forms including combustible organic materials, or organic constituents. Many times the waste is in the form of a slurry or thick sludge which requires drying pretreatment. However, most conventional drying processes do not remove organic constituents, particularly those with low evaporation rates, or they remove but do not destroy the organic constituents. The presence of organic constituents in the vitrification process may reduce metallic ions and inhibit their encapsulation.
Early systems employed the use of incinerators to burn dried waste materials. These incinerators had problems associated with ashes, produced by incomplete combustion, entrained in the off-gas. These airborne ashes thus presented an environmental hazard if not properly treated by filtering systems. Also, problems existed with high amounts of noncombustible leachable inorganic constituents left behind in the ash.
Other systems utilized solidification with concrete, as generally taught, for example, by U.S. Pat. No. 4,504,317 to Smeltzer et al. entitled "Encapsulation of Boric Acid Slurries." Solidification with concrete, however, was in many cases undesirable due to the increase in waste volume and weight. Also, in many cases, the solids in the sludge would interfere with the concrete chemistry and cause weaknesses in the concrete.
Combination incineration-vitrification processes have also been employed to resolve problems associated with use of incinerators alone, as taught, for example, by U.S. Pat. No. 5,022,329 to Rackey et al. entitled "Cyclone Furnace For Hazardous Waste Incineration and Ash Vitrification" and U.S. Pat No. 4,666,490 to Drake entitled "Aqueous Waste Vitrification Process and Apparatus," both of which are incorporated by reference in their entireties herein. These systems entail high fuel and capital costs to provide an ash product to vitrify.
Encapsulation of waste in glass by vitrification is an extensively studied waste encapsulation process, and taught, for example by U.S. Patent No. 4,376,070 to Pope et al. entitled "Containment of Nuclear Waste;" U.S. Pat. No. 4,759,879 to Cadoff et al. entitled "Glass Former Composition and Method for Immobilizing Nuclear Waste Using the Same;" U.S. Pat. No. 5,288,435 to Sachse et al. entitled "Treatment of Radioactive Wastes;" and U.S. Pat. No. 5,416,251 to Lomasney et al. entitled "Method and Apparatus for the Solidification of Radioactive Wastes and Products Produced Thereby;" all of which are incorporated by reference in their entireties herein. However, the presence of combustible organic constituents and water in the waste decreases the viability of vitrification. The vitrification process expends energy to drive off the water, thus driving up the operating costs. Additionally, vitrifiers have a limited heat transfer area and cooling by water evaporation could provide freezing and conductivity problems in joule-heated vitrifiers. This could be designed around by increasing the size of the vitrifiers, but that would increase the capital cost of the vitrifiers. Further, the combustible organic constituents in the waste contribute to increasing the quantity and volume of the vitrifier off-gas stream, requiring a larger off-gas stream treatment facility at increased capital and operational costs. Additionally, the presence of reductive carbon sources in combination with the reducing conditions of most glass melts and with the elevated operating temperatures of the vitrification process results in the metallic ions in the waste being converted to metals via carbothermic reduction reactions. This creates a poor encapsulation in that the metals are not included as readily into the glass matrix as metallic ions, the metal concentration may be higher than the metal solubility concentrations, and the resulting glass product has poor leaching and strength performance.
It is therefore desirable to provide a processing waste method and system that a delivers low-water, low-organic constituents waste stream.